Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components.
A well developed field exists regarding the investment casting of turbine engine parts such as blades and vanes. In an exemplary process, a mold is prepared having one or more mold cavities, each having a shape generally corresponding to the part to be cast. An exemplary process for preparing the mold involves the use of one or more wax patterns of the part. The patterns are formed by molding wax over ceramic cores generally corresponding to positives of the cooling passages within the parts. The patterns are mounted to a shelling fixture. Prior to mounting, the fixture may be prepared to receive the patterns. For example, the fixture may be dipped in wax to at least coat a base plate of the fixture. The wax patterns may be placed atop the wax coating on the base plate and wax welded thereto.
In a shelling process, a ceramic shell is formed around one or more such patterns such as by spraying and/or dipping a ceramic coating material over the fixtured patterns. The wax may be removed such as by melting in an autoclave. The shell may be further processed such as by trimming and sanding of a base surface to flatten the base surface. The shell may be fired to harden the shell. This leaves a mold comprising the shell having one or more part-defining compartments which, in turn, contain the ceramic core(s) defining the cooling passages. The shell may be seeded to define the crystal orientation of the ultimate part and placed with its base surface atop a chill plate in a casting furnace. Molten alloy may then be introduced to the mold to cast the part(s). Upon cooling and solidifying of the alloy, the shell and core may be mechanically and/or chemically removed from the molded part(s). The part(s) can then be machined and treated in one or more stages.
Typically, there are two prevailing investment casting mold design philosophies in the prior art. The first design has a mold dipped in a cage, with a top and bottom plate. The cage is completely immersed in the shell build process. The ceramic top plate is then trimmed with a diamond wheel prior to casting in order to maintain a tight baffle fit necessary in the DS casting process. This mold design has its advantages. For instance, the mold permits bottom feeding with ease and facilitates proper venting. The bottom feeding practice limits the erosion experienced by the ceramic shell and also washes away any debris through the part cavity while filling takes place. However, the trimming process liberates a large amount of ceramic dust. This ceramic dust poses a hazard as the dust could become lodged within the molds and trapped in the thin features being produced in the molded part, thus causing part variations.
The second design utilizes a prefabricated pour cup and only a bottom plate. The mold is only dipped until the ceramic slurry overlaps the existing pour cup. This mold design also exhibits advantages. For instance, the design does not employ a top ceramic plate so the trimming step, mentioned above, is eliminated which in turn generates less ceramic debris. In addition, the absence of the top ceramic plate also creates a repeatable top profile of the mold for a consistent baffle fit. However, the mold design does not permit bottom feeding the part cavity and also fails to leave a passage for wax to evacuate during the venting or dewax process.
Consequently, there exists a need for an investment casting mold design that eliminates the trimming step and limits the amount of debris generated.
There also exists a need for an investment casting mold design that provides a route for the wax to exit during the dewax process.